(589f) Composite Membranes Based on Post-Sulfonated Non-Florinated Aromatic Polymers

Intensive research on development of alternative low cost, high temperature membranes for PEM fuel cells is underway due well known limitations of the industry standard fluorinated membranes. Incorporation of various inorganics into the polymer matrix is a proms©¥ng approach to overcome some these limitations such as decrease in performance at high temperatues (>100 C) and high cost. Post-sulfonated non-fluorinated aromatic polymers such as Polyetheretherketone (PEEK), polyetherethersulfone (PES) and polyarylene ether sulfone (SPES-40) were shown to approach the conductivity of Nafion (DuPont) at high degree of sulfonation (DS) values. However, with increasing DS, particularly at levels needed to achieve similar conductivity to Nafion, it was realized that in some cases the mechanical properties may also diminish with excessive swelling. Although there are accepted Grotthus (proton hopping) and vehicular mechanisms, the proton transport mechanism is still not well established in these materials, but it is obvious that the conductivity is strongly dependent on water content of sulfonated polymers. Inorganic additives at this point serve to hold enough water using their hygroscopic properties without sacrificing mechanical stability or they may also serve to help conduction mechanism as a bifunctional material. Some inorganic fillers studied because of their hygroscopic, conductive or mechanical stability increasing properties are SiO2, TiO2, zeolites, and clays such as laponite and montmorrilonite (MMT). Among these Zeolites are believed to behave as a bifunctional material both both water holding and proton conductivity. The content and synthesis parameters such as Si/Al ratio may be optimized to balance mechanical stability of the membrane and conductivity. In this study the aim was to develop composite membranes having comparable conductivities to Nafion with sufficient mechanical and thermal stability. For this purpose post-sulfonation studies of commercially available engineering thermoplastics PEEK and PES were performed by using concentrated sulfuric acid and chlorosulfonic acid as sulfonating agents. Post sulfonated polymers were characterized with H-NMR, sulfur elemental analysis and titration to calculate DS values and with TGA and DSC for thermal stability and glass transition temperature (Tg). Chemical stabilities were evaluated by hydrogen peroxide tests. Sulfonated PEEK (SPEEK) with DS of 50-70% were used for preparation of composite membranes since it was observed that above this range SPEEK loses its mechanical stability significantly with excessive swelling. It was observed that sulfonation of PES is difficult because of the electrophilic sulfone group in its structure. DS up to ~20% achieved for SPES samples. Various methods are being tried for increasing DS of SPES such as using silica sulfuric acid as a sulfonating agent as a novel method. During sulfonation studies of PES obtained from two different companies, it was found that the sulfonic acid group attached different positions fron H-NMR results. Since one of these polymers include certain ratio of PEES units in it, sulfonic acid groups preferred next to aromatic hydrogen at 7.5 ppm shift and only this portion could be sulfonated whereas in the other PES this was at 8.3 ppm shift. Zeolite Beta (commercial with Si/Al ratio of 75 and 100), and lab synthesized were used as an inorganic fillers. The zeolite Beta formulation used was (2.2Na2O:Al2O3:ySiO2:4.6(TEA)2O:tH2O, where TEA°'tetraethylammoniumhydroxide was prepared from two precursor solutions. X-ray powder diffraction (XRD), TGA, solid state 27Al MAS-NMR and 29Si MAS-NMR were used for characterization. Different Si/Al ratio zeolites up to 30 were synthesized. Composite membranes with laboratory synthesized zeolite, commercial zeolite, TiO2, MMT as inorganic fillers and sulfonated polymers as host polimer matrix were used by solution casting method on glass Petri dishes. N-Methyl pyrrolidone (NMP) was used as the solvent. Membranes were characterized by water uptake test, hydrogen peroxide test, two-probe AC impedance spectroscopy (proton conductivity) with an impedance analyzer (HP-Agilent 4294A-40 Hz to 110 MHz). The proton conductivities were measured at full hydration at room temperature. Some of the membranes were tested in a single cell (5 cm2) test system by using 0.4 mg Pt/cm2 loaded electrodes (5 cm2) at 70 0C. Preliminary results of composite membranes prepared by 10% inorganic content in sulfonated PEEK (DS=60%) are as follows: Water Uptakes (%) and proton conductivity (S/cm) respectively: SPEEK: 15; 2.0*10-2, SPEEK-MMT: 18; 1.6*10-2, SPEEK-TiO2: 21; 2.2*10-2, SPEEK-Zeolite (Si/Al=75): 22; 1.0*10-2. Even high loading of 10% inorganic filler did not caused a decrease in conductivity while increasing mechanical stability. Blends of SPEEK/PES are also tested since the sulfonation of PEEK is relat©¥vely easier, but the mechanical stability decreases at high DS. Sulfonation of PES is more difficult, but may function as a stabilizer. Composite membranes with Zeolite-Beta-(SPES-40) , Nafion-zeolite Beta and Nafion-zeolite 4A resulted in increased conductivity than recast Nafion 117 at low relative humidity.